Reversible phase transition from vesicles to lamellar network structures triggered by chain melting.

Reversible phase structural transition from densely packed multilamellar vesicles of cationic and anionic (catanionic) tetradecyltrimethylammonium laurate (TTAL) with an amount of salt (NaBr) to network structures was triggered by chain melting. Phase behavior of catanionic TTAL multilamellar vesicles in aqueous solutions at different concentrations of NaBr with increasing temperature was studied. This phase structural transition is a progressive process and happens at the chain melting, which was monitored by means of Fourier transform infrared (FT-IR) spectroscopy, turbidity and viscosity measurements. Transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) were used to demonstrate the phase structural conversion from vesicles to three-dimensional structures consisting of extended bilayer networks. We found that the phase transition temperature (Tm) was influenced by adding amount of salt but not by being diluted. This is the first time that the phase conversion from catanionic surfactant vesicles to bilayer networks triggered by chain melting has been observed. The phase structural transition should arise from the enhanced membrane elasticity accompanying the catanionic surfactant state fluctuations on chain melting and the solvent-associated interactions including cationic and anionic surfactant electrostatic interaction, which favors a change in membrane curvature. We hope this phase conversion observed in catanionic surfactants in aqueous solution will provide good insight into the nature of the fusion or fission processes and the fluctuation of catanionic vesicular systems.